U.S. troops who mistakenly killed an Italian agent last week on the road to Baghdad's international airport were part of extra security provided by the U.S. Army to protect U.S. Ambassador John Negroponte (search), a U.S. official said Thursday.

Italian intelligence agent Nicola Calipari (search) was killed when U.S. troops opened fire on a car carrying him and Italian journalist Giuliana Sgrena (search), who had just been freed from insurgents.

"The mobile patrol was there to enhance security because Ambassador Negroponte was expected through," U.S. Embassy spokesman Robert Callahan said.

It was not known if Negroponte had already passed through the checkpoint.

The U.S. Army has launched an investigation into the shooting, which has become a point of contention between the United States and Italy.

Italian Prime Minister Silvio Berlusconi (search) has questioned the U.S. Army's version of events, including a claim that the car was speeding and ignored signals to stop.

The Army has acknowledged the checkpoint was temporary but has provided no details about why it was set up.

The day after the March 4 shooting, a spokesman for the 3rd Infantry Division in Baghdad, U.S. Lt. Col. Clifford Kent, said the checkpoint where the shooting occurred had been set up temporarily and wasn't permanent.

Asked at the time about how easy it would be to see American troops at such a checkpoint a night, he said: "depending on where it is, that could be difficult. But if you're seeing soldiers in military uniform with military equipment, if you know it's a dangerous area, then ... you need to maintain your awareness."